app/views/devise/sessions/new.html.erb
<div class="login">
<h2>Log in</h2>
<%= form_for(resource, as: resource_name, url: session_path(resource_name)) do |f| %>
<% if alert %>
<div class="panel-body">
<div class="text-danger"> <%= alert %> </div>
</div>
<% end %>
<div class="form-group">
<div><%= f.label :email %><br />
<%= f.email_field :email, autofocus: true, class: "form-control" %></div>
</div>
<div class="form-group">
<div><%= f.label :password %><br />
<%= f.password_field :password, autocomplete: "off", class: "form-control" %></div>
</div>
<div class="form-group">
<% if devise_mapping.rememberable? -%>
<div><%= f.check_box :remember_me %> <%= f.label :remember_me %></div>
<% end -%>
</div>
<div class="form-group">
<div><%= f.submit "Log in", class: 'btn btn-primary' %></div>
</div>
<% end %>
</div>